1. Field of the Invention
The present invention relates to a cardiac event detecting system for an implantable heart stimulator intended to be connected to the heart of a patient with at least two unipolar electrode leads, or at least one bipolar electric lead having one electrode pole in the atrium and one electrode pole in the ventricle for sensing heart signals, the detecting system being of the type having at least two signal channels for signals sensed between the two electrode poles and between one of the electrode poles and the stimulator capsule respectively, each signal channel having a signal processing stage, and decision logic supplied with the respective channel signals for comparing the signals from the two signal channels with signal criteria for detecting the occurrence of a cardiac event.
2. Description of the Prior Art
In connection with cardiac pacing there is a general desire to be able to use unipolar electrodes instead of bipolar ones. Bipolar electrodes have two helically wound conductors connected to the electrode tip and ring respectively, which make the electrode more stiff and more frequently defective than a unipolar electrode, which only has one helically wound conductor.
In heart signal detection there are competing requirements. Thus a detecting level must be used for an absolutely reliable detection of the desired heart signals even at amplitude variations due to e.g. normal physiologically dependent amplitude variations, different heart signal origins, and possible variations in the electrode position. Further, a detecting level should be used for an absolutely reliable rejection of undesired signals, such as muscle interferences, far field heart signals and T-waves. A detecting level should also be used for most probable rejection of undesired external interferences. A reliable automatic adaption of the detecting level is also highly desirable.
A large number of different heart signal detectors are known. Normally these detectors include an amplifier with some kind of bandpass filter and a single signal amplitude comparator for determining whether a detection criterion is fulfilled. Certain types of adaption systems have a second "parallel" detector, but even in these latter systems the detection decision is made from a comparison in only one comparator.
Thus in U.S. Pat. No. 5,058,599 a method and an apparatus for detecting a sequence of abnormal events in the depolarization signal of a heart are described. A selective signal parameter is then compared to a defined threshold and the maximum value of the signal parameter is measured for each event which exceeds the threshold. The described method and apparatus are particularly well suited for detecting a sequence of events indicating fibrillation.
Detection of electrical events in the heart by measurements between an electrode pole in the atrium, an electrode pole in the ventricle and the heart stimulator enclosure are also known, see e.g. U.S. Pat. No. 5,607,457, European Applications 0 596 319 and 0 646 390.
U.S. Pat. No. 5,607,457 discloses an evoked response detector with unipolar electrodes in the atrium and the ventricle. A differential detector is connected to the electrode leads and detects cardiac activity between the atrial electrode and ventricular electrode. A correlation detector is connected between the pacemaker housing and one of the atrial or ventricular electrodes to generate a correlation signal identifying whether the detected cardiac activity arose in the atrium or in the ventricle.
European Application 0 596 319 discloses a heart stimulator having an electrode system including at least one bipolar electrode with one pole positioned in the atrium and one pole in the ventricle, or at least two unipolar electrodes arranged in the atrium and the ventricle, respectively. Atrial activity is measured between the two poles of the bipolar electrode or between the two unipolar electrodes, and ventricular activity is measured between the ventricular pole or electrode and the stimulator housing.
Also European Application 0 646 390 describes measurement between an atrial electrode and a ventricular electrode and measurement between the ventricular electrode and the heart stimulator housing for detecting spontaneous and evoked heart responses. One atrial comparator and one ventricular comparator are provided for comparing measured signals with an atrial reference potential and a ventricular reference potential, respectively.
U.S. Pat. No. 4,905,708 discloses an apparatus for recognizing cardiac arrhythmias. Analog signals obtained by sensing at the heart or on the body of the patient are digitized and a first differentiation of the digitized signals is carried out. A gradient pattern detector compares the differentiated signals with normal differentiated signals detected during sinus rhythm. Differences between the differentiated signals are indicative of pathological tachycardia, the nature of which can be identified by comparison with pre-programmed signals of like type.